1. Field of the Invention
This invention relates to windows of the so-called double-hung type, which are characterized by a frame enclosing two sashes, either or both of which are capable of vertical movement so as to open and close the window. If only the lower sash is movable, the window is strictly speaking a single-hung window. However, the term "double-hung window" usually includes such a window also. Since movement is vertical, some sort of counter weight or balance is required in conjunction with the movable sash or sashes.
2. Description of the Prior Art
The double-hung window is probably the most common of the various window types. However, because of the relatively simple technique employed for movement of the sashes, such windows rely on friction for weather tightness when the window is closed. Representative U.S. patents relating to double-hung windows include U.S. Pat. Nos. 4,503,641 to Swan; 4,570,382 to Suess; and 4,580,366 to Hardy.